4-electrode conductivity sensor with zero external field

Abstract

A conductivity sensor configuration which has essentially no external electrical field so that the calibration of the sensor is not affected by the existence of nearby external objects or fouling on the exterior parts of the sensor. Additionally, the sensor is relatively insensitive to electrode fouling or electrode polarization. The sensor includes a tube through which the liquid to be measured flows. An inner electrode plate extends partially along the length of the tube dividing the central portion of the tube into two sections. Two inner electrodes are located on the inner wall, one on each side, and two outer electrodes are located on the inside of the tube opposite the inner electrodes.

What is claimed is:  
     
       1. A conductivity sensor, comprising: 
       a hollow tube made of an insulating material and being of uniform cross-section along at least a portion of its length and open at each end;  
       a divider plate, made of an insulating material, shorter in length than the tube and located midway along the length of the tube, and which extends completely across the tube so as to divide the tube longitudinally into two equal halves on either side of the plate;  
       two inner electrodes respectively located on either side of the divider plate and logitudinally positioned in the center of each side of the plate;  
       two outer electrodes, each located inside the tube in one of said halves on the inside wall opposite an inner electrode; and  
       terminal means for providing an electrical connection to each of the inner and outer electrodes.  
     
     
       2. The conductivity sensor of  claim 1  wherein the distance from the ends of the divider plate to the ends of the tube is greater than twice the distance between the inner and outer electrodes. 
     
     
       3. The conductivity sensor of  claim 1  wherein the distance from the ends of the divider plate to the ends of the tube is it at least four times the distance between the inner and outer electrodes. 
     
     
       4. The conductivity sensor of  claim 1  where the inner and outer electrodes are in the form of stripes, longer than they are wide, and wherein the stripes are oriented orthogonally to axis of the tube. 
     
     
       5. The conductivity sensor of  claim 1  wherein the hollow tube has a rectangular cross-section so that the tube has top and bottom walls and two side walls; and 
       wherein the divider plate is in the form of a rectangular wall running lengthwise through the middle of the tube and extending from the top wall of the tube to the bottom wall.  
     
     
       6. The conductivity sensor of  claim 5  wherein the outer electrodes comprise two conductive stripes applied to each of the side walls of the tube midway along its length; and 
       wherein the inner electrodes comprise two conductive stripes applied to each side of the divider plate midway along its length.  
     
     
       7. The conductivity sensor of  claim 6  wherein the conductive stripes forming the inner and outer electrodes extend from the top wall of the tube to the bottom wall. 
     
     
       8. The conductivity sensor of  claim 7  wherein the dimensions of the tube cross-section and the thickness of the divider plate are such that said equal halves on each side of the divider plate are substantially square in cross-section. 
     
     
       9. A conductivity sensor, comprising: 
       a hollow rectangular tube of uniform cross-section along its length, having top, bottom, and side walls, and being open on each end;  
       a flat electrode plate shorter than the length of the tube, extending from the top wall to the bottom wall of the tube parallel to the side walls, and being located midway between the side walls and midway along the length of the tube;  
       two outer electrodes, each comprising vertical stripe formed on the inside of a respective one of the side walls of the tube midway along its length;  
       two inner electrodes, each comprising a vertical stripe formed on a respective one of the sides of the electrode plate midway along its length; and  
       means for providing electrical connection to each of the electrodes.  
     
     
       10. The conductivity sensor of  claim 9  wherein the distance from the ends of the electrode plate to the ends of the tube is greater than twice the distance between the inner and outer electrodes. 
     
     
       11. The conductivity sensor of  claim 9  wherein the distance from the ends of the electrode plate to the ends of the tube is it at least four times the distance between the inner and outer electrodes. 
     
     
       12. The conductivity sensor of  claim 9  further comprising a slot in the bottom wall of the tube through which the divider plate runs; 
       the electrode plate further comprising a support section which extends through said slot so as to project beyond the tube; and  
       base means attached to the support section of the divider plate for mounting the conductivity sensor.  
     
     
       13. The conductivity sensor of  claim 12  wherein the electrode plate includes two conductive stripes on each side of the support section, the first conductive stripe on each side extending into the tube and forming the inner electrode at its upper end; 
       and further comprising means for connecting the second conductive stripe on each side of the electrode plate to the outer electrode on that side.  
     
     
       14. The conductivity sensor of  claim 13  wherein each of the outer electrodes is an assembly including a conductive stripe on the surface of a mounting plate; and 
       wherein the tube includes a slot on each side for receiving the outer electrode assembly.  
     
     
       15. The conductivity sensor of  claim 14  wherein the base means includes a plug adapted to be received by a receptacle in a pressure-tight relationship; 
       the block including a slot through which the electrode plate extends; and  
       wherein the conductive stripes extend through the slot to provide means for electrical connection to the electrodes from the underside of the plug.  
     
     
       16. A conductivity sensor, comprising: 
       a hollow tube made of an insulating material, open at each end, and having its length as its largest dimension;  
       a divider plate, made of an insulating material, shorter in length than the tube and located inside the tube and partway along the length of the tube so that the tube extends beyond the ends of the divider plate, the divider plate extending completely across the tube so as to divide the tube into two sections on either side of the plate;  
       two inner electrodes respectively located on either side of the divider plate;  
       two outer electrodes, each located on the inner wall of the tube, each electrode being positioned opposite and across from an inner electrode; and  
       terminal means for providing an electrical connection to each of the inner and outer electrodes.  
     
     
       17. The conductivity sensor of  claim 16 , wherein the hollow tube is of rectangular cross section along at least a portion of its length. 
     
     
       18. The conductivity sensor of  claim 16  wherein the inner and outer electrodes are located midway between the ends of the hollow tube. 
     
     
       19. The conductivity sensor of  claim 18  wherein the hollow tube has a symmetrical cross-section, and 
       wherein the divider plate is positioned so as to divide the hollow tube into two equal sections on either side of the plate.